A team of physicists have curbed the hope that quantum physics might be squared with common sense. At least if we want to hang on to Einstein's highly respected theory of relativity. Their result concerns what Einstein called "spooky action at a distance" and it may soon be possible to test their prediction in the lab.

Spooky action at a distance. Image: Timothy Yeo, Centre for Quantum Technologies at National University of Singapore.

Einstein's "spooky action" shows up when we are dealing with so-called entanglement: two particles that
have once interacted can remain linked even when they are
moved light years apart. An example is a property of electrons called spin. When you measure the spin of an electron you will find it pointing either up or down. Now if you measure the spin of one of two entangled electrons and find it pointing up, then immediately the spin of the other, if measured, will be pointing down, no matter how far away it is. It's a strange effect, but it has been demonstrated in the lab many times. (For another example of entanglement and Einstein's criticism of it see this article.)

There are two common sense explanations for what is going on here. One is that something in the past, some cause we have not yet discovered, determined what the spin of the two electrons was going to be. The other is that the two electrons are communicating with each other, sending messages that travel so fast that the interaction appears instantaneous.

The first explanation was ruled out in the 1960s by the physicist John Bell. He showed that if the behaviour of two entangled particles were down to a past cause, then measurements of their properties would have to satisfy a particular inequality, now called Bell's inequality. But both theory and experiment have shown, time and time again, that the inequality doesn't hold: nothing in the past can have determined the particles' properties at the time of measurement.

The second explanation gets into trouble with Einstein's theory of relativity. Experiments have shown that the messages that pass between particles would have to travel more than 10,000 times faster than light. But according to Einstein nothing, and that includes information, can travel faster than light.

This seems to put quantum theory in conflict with relativity, but there is a subtlety which irons out the tension. What relativity forbids is faster-than-light communication. But physicists expect that what happens between entangled particles can't be harnessed for true communication, essentially because there is no way anyone can control the "messages", or influences, that pass between particles. If this is indeed the case, relativity is safe.

Textbook quantum theory holds that the interaction between particles is instantaneous. But what if the
particles interact through influences that do take some time to travel across space? These influences would have to travel faster than light, but that's ok since they don't allow for communication. And the idea would sit much more comfortably with our intuition that a cause must come before its effect.

It is this hidden influences idea that the researchers wanted to explore. "We are interested in whether we can explain the funky phenomena we
observe without sacrificing our sense of things happening smoothly in
space and time," says Jean-Daniel Bancal, one of the physicists
behind the new result.

No experiment can rule out the hidden influence idea directly, so the researchers used a theoretical argument. They imagined a system of four entangled particles and supposed that influences travel between them at finite speed. Like John Bell they came up with an inequality: if the system is to remain un-exploitable for faster-than-light communication, then measurements must satisfy this inequality. They then showed, also using a theoretical argument, that the system does not satisfy the inequality. So if influences do travel at finite speed then faster-than-light communication also becomes possible.

This leaves us trapped between two stark choices. Either we accept that Einstein was wrong about light being a universal speed limit — that's a pill most physicist will find extremely hard to swallow — or we accept that spooky action at a distance is real: influences travel at infinite speed, or at least they appear to do so. In any case, what this means is that parts of the Universe that are arbitrarily far apart are linked in a mysterious instantaneous way.

So far the result is purely theoretical, but the four-particle experiment envisaged by the researchers may become feasible in the near future. If, as predicted, it violates the inequality, and if we want to hold on to relativity, then we must accept that the quantum world is profoundly counter-intuitive.
"Our result gives weight to the idea that quantum correlations somehow arise from outside spacetime, in the sense that no story in space and time can describe them," says Nicolas Gisin, Professor at the University of Geneva, Switzerland, who is a member of the team.